In a raster-based devices such as a television cathode ray tube, the electron beam that excites the colored display phosphors (by “landing” on them) must be precisely directed, since the phosphors are small, particularly in higher resolution devices. Because of variations in the Earth's magnetic field, the orientation of the TV relative to the Earth's field, etc., it can be necessary to correct the beam to land on the center of the intended phosphor during the raster sweep, because variations in the ambient magnetic field can cause the beams to move a small amount from their intended direction. Moreover, beam alignment errors can also be caused by pressure changes such as might be caused by altitude change and also by temperature variations, making it necessary to correct for landing misalignment to account for such pressure- and temperature-based misalignment. When the beams are near the edges of the display they do not misalign much, and when directed near the center of the display do not typically misalign in the plane of the raster sweep, consequently requiring minimal correction in case of altitude-based misalignment. For beam locations not in these areas, however, correction may be necessary. This correction currently is achieved by appropriately establishing the biasing current of several landing correction coils (usually five) inside the display but outside the tube portion of the display, usually located near the corners of the display and also around the perimeter of the tube.
The present invention recognizes that the above-mentioned correction coils can correct beam direction in the corners of the display, where the coils are located, but are less effective in correcting beam direction in locations away from the display corners. The present invention further recognizes that on the neck of many raster-based displays, a so-called velocity modulation (VM) coil is disposed to modulate the horizontal scanning speed of the electron beam. That is, in the neck of the display one or more junctions of the red, green, and blue beams of the display exists, and it is at one of these junctions (the only junction in some cases, e.g., a Sony Trinitron® TV) that the VM coil is located. The present invention further recognizes that in addition to its conventional function, this already existing coil or another coil can be used to correct for landing misalignment.